The present invention relates to a clutch arrangement comprising a shift piston and a piston housing, said shift piston being arranged to be displaced in an axial direction relative to the piston housing between at least one gear position and a neutral position, the clutch arrangement further comprising a spring element for forcing the shift piston towards said neutral position.
The invention is applicable on vehicles, for example working machines within the fields of industrial construction machines, such as wheel loaders and articulated haulers. The invention is however not restricted to these particular machines, but may also be used in other construction equipment or in other vehicles, such as a truck or similar. In particular, the invention is applicable on a dog clutch transmission for transmitting power from a hub motor to a wheel.
Various clutch arrangements are known in order to couple rotating elements to each other so as to transmit power from one element to another element.
In the field of automotive transmission devices, it is known to use a so-called dog clutch, which is an arrangement used to connect two rotating elements, such as for example rotating shafts. More precisely, a dog clutch is arranged for coupling together a set of evenly spaced teeth, so-called “dogs”, on one part of the clutch arrangement to a corresponding set of teeth on the other part of the clutch arrangement. In this manner, the shafts can be coupled together so as to transfer rotational power from one shaft to another.
In contrast to certain other clutches which are based on power transfer via friction between two parts, the dog clutch operates by coupling two parts together without any slip so that they rotate together at the same speed.
Furthermore, an actuating device in the form of a shift piston can be used so as to be displaced for actuating the dog clutch in a manner so that it assumes an engaged or disengaged condition.
In order to engage and lock the two rotating parts of a dog clutch, a so-called synchromesh device can be used in order to match and synchronize the rotational speeds of the rotating shafts before the dog clutch is coupled.
A clutch arrangement is known from the patent document U.S. Pat. No. 4,044,869, which discloses a lamellae disc clutch arrangement for selecting between a forward and a reverse gear position, and also a neutral position. The clutch arrangement is operated by means of a hydraulically operated shift actuator which is arranged in a housing and which can be retained in its neutral position by means of a spring arrangement. The forward and reverse gears can be selected by applying liquid pressure to the shift actuator.
In certain applications in the field of vehicle transmissions, there is a particular need for a dog clutch arrangement which can operate with a highly accurate and precise neutral position. This is important since it may allow for a relatively quick shifting, i.e. a short shifting time when actuating the shift piston so as to shift between gears. There is also a need for a dog clutch arrangement which may fulfil requirements regarding safety, in particular so that it may assume a neutral position in the event of a failure such as a loss of fluid pressure for actuating the shift piston.
It is desirable to provide a clutch arrangement comprising a shift piston by means of which a more precise neutral position and relatively quick shifting times can be obtained.
In accordance with an aspect of the invention, a clutch arrangement comprises a first return piston and a second return piston arranged on opposite sides of said spring element, and said spring element is accommodated within a recess of said shift piston and is pretensioned between the first return piston and the second return piston, wherein in said neutral position the first return piston is forced by the spring element against a first stop element of the shift piston, defining an end position for the first return piston relative to the shift piston, and the second return piston is forced by the spring element against a second stop element of the shift piston, defining an end position for the second return piston relative to the shift piston.
The shift piston carries a pre-tensioned spring which is arranged between said first return piston and said second return piston. In this manner, the spring together with the return pistons strive to position the shift piston in a well-defined neutral position. This means that possible deficiencies or variations in the properties and operation of the spring element do not affect the accuracy or position of the shift position in said neutral position. In other words, a very precise neutral position can be achieved which in turn means that the clutch arrangement according to the invention may be designed so as to allow a very short gear shifting time, which is an advantage in many applications of the invention. The neutral position is defined as a condition in which both the first and second return pistons are forced by said spring element to assume their respective end position. This means that the neutral position can be determined with high accuracy and that other parameters, such as for example the spring force of the spring (springs), are not decisive as regards the positioning and tolerances related to the neutral position.
The spring (or springs) will thus aid in quickly changing from an engaged gear to a neutral position as well as keeping the actuating piston steadily in a neutral position. Also, in certain applications, there is a requirement to provide a dog clutch which allows certain safety functions, for example in a situation in which a mechanical transmission is combined with an electric drive. In such a case, it is necessary to provide a possibility to disengage the dog clutch in an efficient manner if for example an electrical control system malfunctions. According to the invention, the shift piston will move to the neutral position in the event of a loss of pressure of the hydraulic system and is thus easily and securely placed in neutral if the pressure is relieved. This is an important safety feature of the invention.
According to an embodiment, the shift piston is hydraulically actuated and the clutch arrangement is provided with a first inlet for hydraulic fluid extending through said piston housing. In this manner, the shift piston can be forced towards the first axial position by means of the hydraulic fluid. Also, the clutch arrangement can be provided with a second inlet for hydraulic fluid extending through said piston housing and forcing the shift piston towards its second axial position by means of the hydraulic fluid. Also, the embodiment may comprise a third inlet for hydraulic fluid arranged in the piston housing, so that the fluid can be fed into the internal recess in order to force the shift piston towards said neutral position.
The arrangement according to the invention can be applied in different types of clutches but is preferably applied to a dog clutch.